The present invention relates to cooling a metal strip traveling through a cooling section in a continuous heat treatment line, such as an annealing line or a line for applying a metal or an organic coating.
In continuous heat treatment lines of the above-mentioned type, metal strips are cooled in a cooling section by blowing a gas, generally a mixture of nitrogen and hydrogen, through one or more cooling boxes that are fitted with associated holes or blow tubes.
A constant concern of designers of cooling sections lies in cooling the strip traveling through said section as uniformly as possible, while simultaneously avoiding giving rise to instabilities and/or vibration in the traveling strip.
Document EP-A-1 655 383 discloses such a cooling device, in which a strip travels between two cooling boxes fitted with blow tubes inclined at an angle that is directed upstream and/or downstream relative to the traveling strip, and also towards the edges thereof. As the strip passes through the cooling section, it is thus cooled on both faces by the blown-in mixture of gas that is at a temperature lower than the temperature of the strip. The pressure needed for blowing is provided by one or two associated fans. The gas mixture that is heated by heat exchange with the strip is cooled in a heat exchanger, generally a water heat exchanger, so as to be transferred subsequently to a cooling system via the fan(s), thus being recirculated to the cooling boxes.
It is known that heat transfer depends on the blowing distance between the strip and the outlet orifices for the gas mixture, and also on the geometrical configuration of the blowing and the speed of the blowing. It is known that heat transfer is more effective when the blowing distance is small and/or the blowing speed is high. Nevertheless, there are practical limits on increasing the blowing speed and on reducing the distance between the strip and the blowing system, since beyond a certain threshold vibration and/or oscillation of the strip appears, and that can lead to the strip coming into contact with the blowing system, thereby leading to marks that are incompatible with the desired surface quality, and possibly even damaging the strip more severely.
In a variant to the technique of blowing a gas mixture, water has also been used as a cooling fluid, as disclosed in document EP-A-0 343 103, in which the strip is cooled rapidly by means of nozzles delivering a water/air mist, or in a variant as disclosed in FR-A-2 796 965 in which water/nitrogen nozzles are used.
The use of water as a cooling fluid is advantageous insofar as heat transfer requires lower outlet speeds for the cooling fluid, since transfer is based on exchanging heat by evaporating the water into air or nitrogen, however that technique presents two major drawbacks. The first drawback is that heat transfer is limited by the saturation temperature of water in the incondensable air or nitrogen gas, and the second drawback is that steel at high temperature inevitably suffers oxidation when cooled by a mist of water and air or of water and nitrogen, which means that it is subsequently necessary to perform special treatment for removing an oxide film, where such treatment can be expensive and sometimes even impossible to perform on certain lines such as galvanizing lines.
There thus exists a need for a cooling method that provides better performance, being capable of significantly increasing the speed at which a traveling metal strip is cooled, but without that setting the strip into vibration and/or oscillation, and without causing said strip to oxidize.